Many output buffer circuits are coupled to one or more power supplies and use output drivers to switch an output voltage according to the values of inputs. For example, an output buffer that receives a high voltage (VDDHV) from a first power supply and an internal chip core voltage (VDD) from a second power supply might have p-channel and n-channel output drivers to switch the voltage of an output according to values of a data input and an enable input. It is often desirable to protect the gate oxides of these output drivers and other components of the output buffer from overstress, break-down, or other damage due to changes in voltage on the output.
As microelectronic devices become increasingly complex to satisfy additional processing requirements, reducing the failure of devices during operation becomes increasingly important. A known technique for protecting the gate oxide of an n-channel output driver includes coupling a cascode device between the output driver and the output to shield the gate oxide of the output driver from changes in voltage on the output. The gate of the cascode is maintained at a substantially constant intermediate voltage, such that the voltage across the gate oxide of the output driver does not exceed the gate oxide's maximum fixed voltage tolerance. Although these techniques protect the output driver gate oxide when the second power supply is supplying VDD to the output buffer, if the second power supply is not powered up, the second power supply fails, or the supply of VDD to the output buffer is interrupted for any other reason, the gate oxide of the output driver may be overstressed, broken down, or otherwise damaged due to voltage changes on the output. Furthermore, previous output buffers may use bias generators that take up space, draw current, and unduly complicate the output buffer. These and other inadequacies make prior output buffers unsuitable for many important applications.